Comparative analysis of salt cleaning and osmotic backwash on calcium-bridged organic fouling in nanofiltration process

The cleaning efficiency of salt cleaning and osmotic backwash in a nanofiltration (NF) process is evaluated using model organic foulants in the presence of calcium ions (Ca²⁺). In particular, the organic fouling behavior—which depends on Ca²⁺ concentration—and cleaning mechanism in both salt cleaning and osmotic backwash are studied. The results show that Ca²⁺ aggravates organic fouling, and the fouling formation rate is affected by the Ca²⁺/foulant ratio. Salt cleaning and osmotic backwash effectively clean Ca²⁺-bridging fouling, and the important cleaning mechanism in both methods is ion-exchange between the monovalent and divalent cations, resulting in the breakup of Ca²⁺–foulant binding and Ca²⁺-induced intermolecular bridging. The cleaning efficiency of osmotic backwash is 26% higher than that of salt cleaning, primarily owing to that the backwash flux creates synergy effect with ion-exchange to removes the weakened fouling structure on the membrane surface. In addition, the cleaning efficiency for irreversible fouling is of the order of osmotic backwash > chemical cleaning > salt cleaning, and osmotic backwash exhibited a 7.3% and 20.8% higher cleaning efficiency than chemical cleaning and salt cleaning, respectively. Therefore, osmotic backwash can stably maintain NF membrane performance for organic fouling and potentially replace the traditional chemical cleaning.